Autoimmunity affects nearly ten million Americans and, for unknown reasons, is increasing in frequency. Available evidence indicates that autoimmune diseases arise due to a combination of genetically determined susceptibility and innate immune stimulation, which conspire to stimulate adaptive immunity to self. Underpinning autoimmunity is the breakdown of immune tolerance to self in T cell and/or B cell compartments. While a great deal is known about the mechanisms that maintain B cell tolerance, we understand little about the mechanisms that cause it to fail in autoimmunity. In this application we propose to study mechanisms operative in loss of B cell tolerance to insulin in murine and human Type 1 Diabetes, a disease known to require B cells that are thought to function by antigen presentation to CD4 T cells. Our approach will build upon our previous work defining signaling pathways operative in maintaining antigen unresponsiveness of anergic B cells. We will analyze by flow cytometry changes in the status of insulin-specific B cells isolated using a novel magnetic bead-based approach from normal, prediabetic and diabetic mice and patients. In some cases immunoglobulin heavy chain transgenesis will be used to generate a diverse antigen receptor repertoire that is nonetheless enriched in insulin-specific cells. Aim 1 will explore how peripheral insulin-specific B cells are silenced in normal mice and whether this silencing is breached in diabetic mice. In Aim 2 we will study the role of antigen receptor affinit for insulin in determining mode of silencing, fate and diabetogenic potential. This will involve a retrotransgenic approach to generate a repertoire with defined antigen receptor affinity for insulin. In Aim 3 we will extend studies to the human, comparing insulin-specific B cells in prediabetic and diabetic individuals to non-diabetic first- degree relatives. Finally, in Aim 4 we will assess the therapeutic efficacy in T1D of a novel B cell desensitizing therapy, comparing this therapy to anti-CD20, which depletes B cells. The experiments will address the overarching hypothesis that in Type 1 Diabetes the silence of high affinity insulin-specific B cells is broken and these cells promote disease by presentation of autoantigen to CD4 T cells.